Bi-SUBSTITUTED RARE EARTH IRON GARNET SINGLE CRYSTAL, METHOD OF MANUFACTURING THE SAME, AND OPTICAL DEVICE

ABSTRACT

Provided is a Bi-substituted rare earth iron garnet single crystal which has a composition of R 3-x Bi x Fe 5-w A w O 12 , wherein R denotes one or more rare earth elements among Y, Eu, Gd, Ho, Yb, Lu, Nd, Tm, La, Sm, Dy, Er, Ce, and Pr while definitely including Gd, A denotes one or more elements among Ga, Al, In, Sc, Co, Ni, Cr, V, Ti, Si, Ge, Mg, Zn, Nb, Ta, Sn, Zr, Hf, Pt, Rh, Te, Os, Ce, and Lu, 0.7&lt;x≦1.5, and 0&lt;w≦1.5, does not contain Pb and contains Pt, and additionally contains M which denotes Mn or at least one Group 2 element, wherein a coefficient α is set to any value within a numerical range of 0.815±0.035 and Δ (which means (α×[M])−[Pt]) is set from −0.40 atppm to 3.18 atppm.

TECHNICAL FIELD

The present invention relates to a lead-free Bi-substituted rare earthiron garnet single crystal, a method of manufacturing the same, and anoptical device including the Bi-substituted rare earth iron garnetsingle crystal.

BACKGROUND ART

A Faraday rotor used in an optical isolator, an optical circulator, orthe like uses a Bi-substituted rare earth iron garnet single crystal(hereinafter, referred to as a “garnet single crystal” as occasiondemands) that is grown according to a liquid phase epitaxy (LPE) method.The LPE method is a method of growing a single crystal, wherein a meltis prepared by putting a material in a crucible and melting the materialand a garnet single crystal is epitaxial-grown by contacting a growingsubstrate with the melt. Since PbO is used in the melt along with B₂O₃,Bi₂O₃, or the like, Pb (lead) contained in the grown garnet singlecrystal is unavoidable.

Recently, in order to prevent an adverse effect of a highly harmfulchemical material on the environment, there have been movements torestrict harmful material from being included in a product worldwide,and lead is also designated as a harmful material. An example of arestriction on harmful materials includes Restriction of HazardousSubstances (RoHS) enforced by the European Union (EU). Moreover, even asmall amount of lead included in a garnet single crystal may causeenvironmental contamination, and this has been a problem.

Thus, in order to remove lead included in a garnet single crystal, it isrequired to grow a garnet single crystal from a melt that does notinclude lead. Specifically, in a garnet single crystal included in anoptical isolator, a thickness for rotating a plane of polarization oflight to about 45° was required for optical communication in awavelength from about 1.3 μm to about 1.6 μm based on a relationship ofa Faraday's rotation angle.

Also, along with such a predetermined thickness, it is important toreduce an insertion loss (hereinafter, referred to as “IL”), which isobtained by converting into the thickness generating the Faraday'srotation angle of 45°, to 0.10 dB or lower, which is a market demand foran optical isolator. When a garnet single crystal is grown from aconventional lead-containing melt, platinum (Pt) is used in a cruciblefor melting a material considering heat resistance and corrosionresistance. Here, when the garnet single crystal is grown in anatmosphere, platinum that is a crucible material is oxidized anddissolved in the melt, and a small amount of platinum is also includedin the grown garnet single crystal as quadrivalent platinum ions (Pt⁴⁺).

Then, it is assumed that bivalent lead ions (Pb²⁺) and quadrivalentplatinum ions (Pt⁴⁺) included in the garnet single crystal generateoptical absorption, and bivalent or quadrivalent iron ions (Fe²⁺ orFe⁴⁺) induced by the bivalent lead ions and quadrivalent platinum ionsincrease optical absorption in a wavelength band from 1.3 μm to 1.6 μm,thereby increasing IL.

Since the garnet single crystal is stable in trivalence, chargecompensation is performed by using quadrivalent additives when bivalentimpurities are excessive, and charge compensation is performed by usingbivalent additives when quadrivalent impurities are excessive. It isthought that since the conventional garnet single crystal contains Pb,the Pb exists in the garnet single crystal as bivalent ions (Pb²⁺) andquadrivalent ions (Pb⁴⁺), and thus a contained balance of the bivalentions (Pb²⁺) and the quadrivalent ions (Pt⁴⁺ and Pb⁴⁺) in the entiregarnet single crystal is almost equally maintained. As a result, thegeneration of Fe²⁺ or Fe⁴⁺, which increases optical absorption, is lowin the conventional garnet single crystal, and thus it is thought thatIL is suppressed from being increased (assuming that Pb²⁺ content >Pb⁴⁺content).

However, since a melt that does not include lead does not contain Pb²⁺and Pb⁴⁺ at all, Pt⁴⁺ is relatively excessive in a garnet single crystalgrown from the melt. Accordingly, Fe²⁺ ions, which are bivalent ions,are generated to perform charge compensation on the excessive Pt⁴⁺, andthus optical absorption is increased, thereby increasing IL.

Accordingly, a method of using gold (Au), instead of platinum, as acrucible material while growing a garnet single crystal from a melt thatdoes not include lead has been suggested. For example, Patent Reference1 suggests adding an alkali metal such as Na, instead of Pb, as a meltcomponent that does not include lead.

Also, a technology of suppressing an increase in IL by adding Ca that isbivalent like Pb²⁺, instead of Pb²⁺, has been suggested (for example,refer to Patent Reference 2).

PRIOR ART REFERENCE

Patent Reference

-   (Patent Reference 1) Japanese Laid-Open Patent Publication No.    2006-169093 (pages 5-9)-   (Patent Reference 2) Japanese Laid-Open Patent Publication No.    2007-153696 (pages 3-7)

DISCLOSURE OF THE INVENTION Technical Problem

However, in a garnet single crystal grown according to a manufacturingmethod disclosed in Patent Reference 1, IL of a Faraday rotor mayincrease according to a is content of Na. Also, in Patent Reference 1,since gold is mainly used as a material for a crucible or a stirringjig, Pt⁴⁺ is not included in the garnet single crystal and opticalabsorption by Pt⁴⁺ is not generated, but a platinum material needs to beused in an LPE method in terms of heat resistance and mechanicalstrength. Also, Patent Reference 1 discloses the manufacturing methodwhere Na is added by using NaOH, but due to concerns of burns orblindness, it is difficult to handle NaOH.

Also, in a manufacturing method disclosed in Patent Reference 2, sincecrystal growing conditions are changed when even a small amount of Ca isincluded in a Bi₂O₃-B₂O₃ solvent, thereby deteriorating a growth speedof crystals, which is important in mass productivity, it was difficultto manufacture a garnet single crystal that does not include lead at allas a mass production base. Also, a quality of a garnet single crystalmay be deteriorated or IL may be increased according to a content of Ca.

Based on such a technical problem, the present invention provides agarnet single crystal that does not contain lead while using a crucibleformed of a platinum material, does not have quality deterioration andhas high mass productivity, and enables IL to be less than or equal to0.10 dB.

Technical Solution

Fundamental concepts of the present invention are to input Mn or atleast one Group 2 element that is bivalence such as Pb so as to performcharge compensation with Pt⁴⁺ that is excessive due to no lead in amelt, and to reduce IL, obtain mass productivity, and promote qualityimprovement based on a relationship between an IL value of a garnetsingle crystal including Gd without lead and a difference between Ptconcentration and concentration of Mn or at least one Group 2 element(Be, Mg, Ca, Sr, Ba, and Ra) included in the garnet single crystal.

According to an aspect of the present invention, there is provided aBi-substituted rare earth iron garnet single crystal which has acomposition of R_(3-x)Bi_(x)Fe_(5-w)A_(w)O₁₂, wherein R denotes one ormore types of rare earth elements selected from a group consisting of Y,Eu, Gd, Ho, Yb, Lu, Nd, Tm, La, Sm, Dy, Er, Ce, and Pr while definitelyincluding Gd, A denotes one or more types of elements selected from agroup consisting of Ga, Al, In, Sc, Co, Ni, Cr, V, Ti, Si, Ge, Mg, Zn,Nb, Ta, Sn, Zr, Hf, Pt, Rh, Te, Os, Ce, and Lu, 0.7<x≦1.5, and 0<w≦1.5,does not contain Pb and contains Pt, and additionally contains Mn or atleast one Group 2 element, wherein, is when M denotes Mn or at least oneGroup 2 element, [M] denotes M concentration (atppm) and [Pt] denotes Ptconcentration (atppm) in the Bi-substituted rare earth iron garnetsingle crystal, and a relational expression A of [M] and [Pt] isrepresented by Equation 1 below:

Δ=(α×[M])−[Pt],  [Equation 1]

wherein a coefficient α is set to any value within a numerical range of0.815±0.035 and A is set from −0.40 atppm to 3.18 atppm.

Also, according to an embodiment of the Bi-substituted rare earth irongarnet single crystal of the present invention, Δ may be set from −0.20atppm to 1.21 atppm.

Also, according to another embodiment of the Bi-substituted rare earthiron garnet single crystal of the present invention, Δ may be set to 0atppm.

According to another aspect of the present invention, there is providedan optical isolator, an optical circulator, an optical attenuator, aFaraday mirror, a current sensor, a magnetic field sensor, and amagnetic optical switch including the Bi-substituted rare earth irongarnet single crystal.

According to another aspect of the present invention, there is provideda method of manufacturing a Bi-substituted rare earth iron garnet singlecrystal, the method including: putting a solvent which does include atleast and does not include a lead compound, and does include Fe₂O₃,Gd₂O₃, and a solute aside from Fe₂O₃ and Gd₂O₃ into a crucible formed ofa Pt material; putting a mixture of any one of MO, MO₂, and M₂O₃,wherein M denotes Mn or at least one Group 2 element, into the solvent;and growing a Bi-substituted rare earth iron garnet single crystal on anonmagnetic garnet crystal substrate, wherein, in the Bi-substitutedrare earth iron garnet single crystal, when [M] denotes M concentration(atppm) and [Pt] denotes Pt concentration (atppm) in the Bi-substitutedrare earth iron garnet single crystal, and a relational expression Δ of[M] and [Pt] is represented by Equation 2 below:

Δ=(α×[M])−[Pt],  [Equation 2]

wherein a coefficient α is set to any value within a numerical range of0.815±0.035 and Δ is set from −0.40 atppm to 3.18 atppm.

Also, according to an embodiment of the method of the present invention,Δ may be set from −0.20 atppm to 1.21 atppm.

Also, according to another embodiment of the method of the presentinvention, Δ may be set to 0 atppm.

Also, according to another embodiment of the method of the presentinvention, the Bi-substituted rare earth iron garnet single crystal maybe grown under an inert gas atmosphere.

Advantageous Effects

A Bi-substituted rare earth iron garnet single crystal according to thepresent invention does not adversely affect the environment since leadis not included in a crystal. Also, by manufacturing a Bi-substitutedrare earth iron garnet single crystal of claim 1 by using a method ofclaim 4, the Bi-substituted rare earth iron garnet single crystalcontains Mn or at least one Group 2 element while optical Mn or Group 2element concentration with respect to Pt concentration is set to a Δvalue from −0.40 atppm to 3.18 atppm. Accordingly, a generation of Fe²⁺(and Fe⁴⁺) in the Bi-substituted rare earth iron garnet single crystalincluding Gd is suppressed, and thus it is possible to reduce opticalabsorption and maintain IL to be less than or equal to 0.10 dB in awavelength of light from about 1.3 μm to about 1.6 μm. Also, bymaintaining the Pt concentration to be less than or equal to the Mn orGroup 2 element concentration, a steep increase of IL in the wavelengthof light from about 1.3 μm to about 1.6 μm may be suppressed.

Also, by manufacturing a Bi-substituted rare earth iron garnet singlecrystal of claim 2 by using a method of claim 5 and setting a Δ value tobe from −0.20 atppm to 1.21 atppm, it is possible to reduce opticalabsorption by suppressing generation of Fe²⁺ (and Fe⁴⁺) in theBi-substituted rare earth iron garnet single crystal including Gd andmaintain IL to be less than or equal to 0.05 dB in a wavelength of lightfrom about 1.3 μm to about 1.6 μm. In addition, by maintaining the Ptconcentration to be less than or equal to the Mn or Group 2 elementconcentration, a steep increase of IL in the wavelength of light fromabout 1.3 μm to about 1.6 μm may be suppressed.

Also, by manufacturing a Bi-substituted rare earth iron garnet singlecrystal of claim 3 by using a method of claim 6 and setting Ptconcentration and Mn or Group 2 element concentration such that a Δvalue is 0 atppm, it is possible to reduce optical absorption bysuppressing generation of Fe²⁺ (and Fe⁴⁺) in the garnet single crystalincluding Gd and set IL to a minimum value in a wavelength of light fromabout 1.3 μm to about 1.6 μm.

Also, according to a method of manufacturing a Bi-substituted rare earthiron garnet single crystal of claim 7, since an inert gas is used for agrowing atmosphere, an increase of Pt⁴⁺ is prevented as oxidation of Ptin a melt is suppressed during each method described above, and it ispossible to suppress generation of Fe²⁺ (and Fe).

Also, according to each optical device of claim 8, the optical devicedoes not adversely affect the environment by including theBi-substituted rare earth iron garnet single crystal of any one ofclaims 1 through 3, since the Bi-substituted rare earth iron garnetsingle crystal included in each optical device does not include lead.

BEST MODE FOR CARRYING OUT THE INVENTION

Pt⁴⁺ from platinum that is a crucible material may be contained in amelt as described above, as quadrivalent ions. Also, accompanied by nolead in a Bi-substituted rare earth iron garnet single crystal(hereinafter, simply referred to as “garnet single crystal” as occasiondemands), Pt⁴⁺ is relatively excessive if there is no Pb²⁺ and Pb⁴⁺ atall. As a result, it is thought that Fe²⁺ is generated to generateoptical absorption, and thus IL is increased. Thus, the inventors addedMn²⁺ as other bivalent ions, instead of Pb²⁺, so as to promote acontained balance of excessive Pt⁴⁺ and bivalent ions in the garnetsingle crystal, and made a close study to reduce IL by suppressinggeneration of Fe²⁺ (and Fe⁴⁺) while maintaining the contained balance.

Consequently, the inventors completed the present invention bydiscovering a relationship of an IL value of the garnet single crystalwith respect to a concentration difference between Pt concentration andconcentration of Mn or at least one Group 2 element (Be, Mg, Ca, Sr, Ba,and Ra), which is put into the garnet single crystal.

Hereinafter, embodiments of the present invention will be described. Agarnet single crystal of the present invention is grown by using an LPEmethod, uses B(B₂O₃) or Bi(Bi₂O₃) as a solvent while using Bi(Bi₂O₃),Gd(Gd₂O₃), Ho(Ho₂O₃), Fe(Fe₂O₃), Ga(Ga₂O₃), or Al(Al₂O₃) as a garnetcomponent that is a solute. Here, a lead compound (PbO or PbF₂) is notused as a solvent.

The garnet single crystal according to the present invention isrepresented by an empirical formula of R_(3-x)Bi_(x)Fe_(5-w)A_(w)O₁₂. Biis an important component of the garnet single crystal and operates bothas a solvent and a solute. Also, an oxide of any one of various rareearth elements, Fe₂O₃, and an oxide of an element substitutable with Feis used as a main solute (garnet component). A rare-earth element R inthe present application may be one or more types of elements selectedfrom a group consisting of Y, Eu, Gd, Ho, Yb, Lu, Nd, Tm, La, Sm, Dy,Er, Ce, and Pr, which are independently stable and are capable ofpreparing the garnet single crystal with Fe. Here, Gd is selected andincluded without fail. Also, an element A substitutable with Fe may beone or more types of elements selected from a group consisting of Ga,Al, In, Sc, Co, Ni, Cr, V, Ti, Si, Ge, Mg, Zn, Nb, Ta, Sn, Zr, Hf, Pt,Rh, Te, Os, Ce, and Lu.

Also, not to include Pb at all, bivalent Mn ions (Mn²⁺) are added to thesolvent in a suitable amount instead of Pb²⁺. By adding the suitableamount of bivalent Mn ions (Mn²⁺), Mn²⁺ and Pt⁴⁺ are charge-compensatedwith each other to maintain an ion balance in the garnet single crystaland to prevent generation of optical absorption generated as Fe³⁺ ischanged to Fe²⁺ or Fe⁴⁺, thereby suppressing IL.

Since other bivalent ions exist as well as Mn²⁺ and are substituted forcomponent ions of the garnet single crystal, the other bivalent ions maybe bivalent ions having an ionic radius close to the component ions. Thegarnet single crystal is grown by injecting CaO, MgO, MnO, MnO₂, Mn₂O₃,or the like as an oxide generating ions into a material. Alternatively,any one of Group 2 elements may be used.

A Bi content in the garnet single crystal affects a Faraday rotationcoefficient, and thus a Faraday rotation coefficient increases as a Bicontent increases. However, when the Bi content exceeds 1.5 in theempirical formula, the Bi content is unable to match a lattice constantwith a growing substrate, and thus a crystal may be defected or crack.When the Bi content is less than or equal to 0.7, a Faraday effect isdecreased, and thus a sufficient Faraday rotation coefficient (deg/cm)is not obtained and a thickness of the garnet single crystal requiredfor 45° rotation in a wavelength of 1.5 μm exceeds 500 μm. An increaseof the thickness is not suitable since a growing time of the garnetsingle crystal may increase and a crystal may be defected or crack. Inthis regard, a Bi content x in the empirical formula is in a range of0.7<x≦1.5.

As Fe is substituted by Ga, Al, In, or the like, the Faraday rotationcoefficient (deg/cm) is decreased and the thickness required for 45°rotation is increased. An Fe substitution amount in the empiricalformula is 0<w≦1.5.

Next, an example of a method of manufacturing a garnet single crystalaccording to the present invention will now be described, but the methodis not limited thereto.

First, a crucible formed of a Pt material is prepared, and oxide powderof elements of a solvent and a solute (garnet component) are put intothe crucible. Boron oxide (B₂O₃) as the solvent, and powder of each ofgadolinium oxide (Gd₂O₃), bismuth oxide (Bi₂O₃), holmium oxide (Ho₂O₃),gallium oxide (Ga₂O₃), alumina (Al₂O₃), and iron oxide (Fe₂O₃) of agarnet material component (solute), as an example of a metal oxideforming an epitaxial film of the garnet single crystal, are mixed, putinto the platinum crucible, heated, and melted. In addition, powder ofany one of MO, MO₂, and M₂O₃ is mixed as an additive of the solvent.Here, M denotes Mn or at least one Group 2 element. A composition of amelt may vary. Here in the present invention, a lead compound (leadoxide (PbO) or the like) used as a conventional solvent is not input.

Next, the crucible is heated up to 1000 to 1200° C. to melt the powderof each of the solvent and the solute during sufficient stirring, andthe melt is uniformly mixed and then a temperature of the melt isdecreased to 700 to 950° C. so that the melt is in a supercooled state.A nonmagnetic garnet crystal substrate (hereinafter, referred to as anSGGG substrate) of a surface orientation {111} having a diameter of 76to 85 mm, a thickness of 0.5 mm, a lattice constant from 12.475 to12.515 Å, and a composition of (GdCa)₃(GaMgZr)₅O₁₂ or Nd₃Ga₅O₁₂ wascontacted on a liquid surface of the melt during rotation, and thegarnet single crystal was grown on the {111} surface via an LPE methodat about 810° C. under an inert gas atmosphere. A growth speed was about0.2 μm/min or more. An inert gas may be, for example, a Group 18element, nitrogen, or the like.

The inert gas is used in the growth atmosphere to prevent an increase ofPt⁴⁺ by suppressing oxidation of Pt in the melt and to suppressgeneration of Fe²⁺ (and Fe). Accordingly, the inert gas is not limitedas long as it is an inert gas atmosphere suppressing Pt from beingoxidized in the melt.

Under such an atmosphere, the garnet single crystal is grown up to apredetermined thickness (for example, about 500 μm). Also, a crystallattice constant of the grown garnet single crystal was 12.488 Ådependent upon the lattice constant of the SGGG substrate. The SGGGsubstrate was removed from the garnet single crystal having such alattice constant and having a composition represented by(GdHoBi)₃(FeGaAl)₅O₁₂, and the garnet single crystal was polished to athickness in a range from 232 μm to 390 μm. Then, an anti-reflectionfilm was deposited on both surfaces of the garnet single crystal, and ILcharacteristics were evaluated.

Upon evaluating a plurality of garnet single crystals, the inventorsderived that a difference between Pt concentration and concentration ofMn or at least one Group 2 element contained in the garnet singlecrystal grown from the melt obtained by mixing the powder of any one ofMO, MO₂, and M₂O₃ with the solvent can be used as a parameter forsetting an IL value of the garnet single crystal to a desired value, byrelating the difference according to Equation 3 below:

Δ=(α×[M])−[Pt]  [Equation 3]

In Equation 3, [M] denotes M concentration (atppm) in the garnet singlecrystal, and [Pt] denotes Pt concentration (atppm) in the garnet singlecrystal. Also, as described above, M denotes Mn or at least one Group 2element.

A coefficient α in Equation 3 is set to be any value in a numericalrange of 0.815±0.035. In other words, the coefficient α is set to be anyvalue in a numerical range from 0.780 to 0.850. In the presentinvention, a reason for setting a variation range of ±0.035 is asfollows. When a Bi-substituted rare earth iron garnet single crystal isanalyzed via a desired analysis method, such as an ICP-MS analysismethod, a value of the coefficient α deviates due to analysis precisionof the analysis method. Thus, considering the deviation, the variationrange of ±0.035 is set.

In the garnet single crystal including Gd according to the presentinvention, optimum Mn or Group 2 element concentration with respect toPt concentration is set to a Δ value from −0.40 atppm to 3.18 atppm, andthus optical absorption is reduced by suppressing generation of Fe²⁺(and Fe⁴⁺) in the garnet single crystal including Gd and it is possibleto set IL in the wavelength of light in a range of 1.3 μm to 1.6 μm tobe less than or equal to 0.10 dB. Also, by setting the Pt concentrationto be less than or equal to the Mn or Group 2 element concentration, asteep increase of IL in the wavelength of light in the range from 1.3 μmto 1.6 μm may be suppressed.

When the coefficient α is set to any value in the range of 0.815±0.035(that is from 0.780 to 0.850), the Δ value may be set from −0.40 atppmto 3.18 atppm. In other words, the Bi-substituted rare earth iron garnetsingle crystal where the Δ value is set from −0.40 atppm to 3.18 atppmwhen the coefficient α has any one value in the numerical range of0.815±0.035 is the Bi-substituted rare earth iron garnet single crystalaccording to the present invention. Also, in the Bi-substituted rareearth iron garnet single crystal having the Δ value from −0.40 atppm to3.18 atppm, a most preferable Bi-substituted rare earth iron garnetsingle crystal is a Bi-substituted rare earth iron garnet single crystalthat always has the Δ value from −0.40 atppm to 3.18 atppm even when thecoefficient α is set to any value within the numerical range of0.815±0.035 (that is, from 0.780 to 0.850).

Also, more preferably, the Δ value is set to be from −0.20 atppm to 1.21atppm. By setting the Δ value as such, optical absorption is reduced bysuppressing generation of Fe²⁺ (and Fe⁴⁺) in the garnet single crystalincluding Gd and it is possible to set IL to be less than or equal to0.05 dB in the wavelength of light from 1.3 μm to 1.6 μm. Also, bysetting the Pt concentration to be less than or equal to the Mn or Group2 element concentration, the steep increase of IL in the wavelength oflight from 1.3 μm to 1.6 μm may be suppressed.

When the coefficient α is set to any value in the numerical range of0.815±0.035 (that is, from 0.780 to 0.850), the Δ value may be set from−0.20 atppm to 1.21 atppm. In other words, the Bi-substituted rare earthiron garnet single crystal where the Δ value is set from −0.20 atppm to1.21 atppm when the coefficient α is any one value in the numericalrange of 0.815±0.035 is the Bi-substituted rare earth iron garnet singlecrystal according to the present invention. Also, in the Bi-substitutedrare earth iron garnet single crystal having the Δ value from −0.20atppm to 1.21 atppm, a most preferable Bi-substituted rare earth irongarnet single crystal is a Bi-substituted rare earth iron garnet singlecrystal that always has the Δ value from −0.20 atppm to 1.21 atppm evenwhen the coefficient α is set to any value within the numerical range of0.815±0.035 (that is, from 0.780 to 0.850).

More preferably, the Δ value is set to be 0 atppm. In other words, thePt concentration and the Mn or Group 2 element concentration multipliedby the coefficient α is the same. By setting the Pt concentration andthe Mn or Group 2 element concentration such that Δ value is 0 atppm,optical absorption is reduced by suppressing generation of Fe²⁺ (andFe⁴⁺) in the garnet single crystal including Gd and it is possible toset IL to be a minimum value in the wavelength of light from 1.3 μm to1.6 μm.

The Δ value may be set to be 0 atppm when the coefficient α is set toany value within the numerical range of 0.815±0.035 (that is, 0.780 to0.850). In other words, the Bi-substituted rare earth iron garnet singlecrystal where the Δ value is set to 0 atppm when the coefficient α hasany one value in the numerical range of 0.815±0.035 is theBi-substituted rare earth iron garnet single crystal according to thepresent invention.

Examples 1 through 8

Hereinafter, embodiments of the garnet single crystal according to thepresent invention, which are manufactured according to the method areshown, but are not limited thereto. Table 1 shows each empiricalformula, Mn concentration (atppm), Pt concentration (atppm), Δ value(atppm), and IL value (dB) in a center wavelength of 1.55 μm of aGd-based garnet single crystal as an example of the garnet singlecrystal according to the present invention. Here, the coefficient α isunified to 0.815, and the third places of decimals of Δ values (atppm)are rounded off. Sample numbers are sequentially given from Examples 1through 8 from the top in Table 1. Also, composition analysis, and Mnconcentration and Pt concentration analysis of the manufactured garnetsingle crystals are performed via an ICP-MS analysis method.

TABLE 1 Mn Pt Concentration Concentration Sample No. Empirical Formula(atppm) (atppm) Δ (atppm) IL (dB) Example 1 (GdHoBi)₃(FeGaAl)₅O₁₂ 6.422.10 3.13 0.097 Example 2 (GdHoBi)₃(FeGaAl)₅O₁₂ 3.09 2.90 −0.38 0.099Example 3 (GdHoBi)₃(FeGaAl)₅O₁₂ 3.28 2.88 −0.21 0.053 Example 4(GdHoBi)₃(FeGaAl)₅O₁₂ 3.09 2.67 −0.15 0.046 Example 5(GdHoBi)₃(FeGaAl)₅O₁₂ 3.46 1.54 1.28 0.052 Example 6(GdHoBi)₃(FeGaAl)₅O₁₂ 3.35 1.54 1.19 0.048 Example 7(GdHoBi)₃(FeGaAl)₅O₁₂ 2.18 1.77 0.01 0.032 Example 8(GdHoBi)₃(FeGaAl)₅O₁₂ 2.61 2.123 0.00 0.020 Comparative(GdHoBi)₃(FeGaAl)₅O₁₂ 2.55 2.58 −0.50 0.171 Example 1 Comparative(GdHoBi)₃(FeGaAl)₅O₁₂ 6.72 2.05 3.43 0.124 Example 2

Based on the results of Examples 1 through 3 and 5, it was checked thatit is possible to set IL to be less than or equal to 0.10 dB by settingthe Δ value to be 3.13 atppm, −0.38 atppm, −0.21 atppm, and 1.28 atppm,which are within a range from −0.40 atppm to 3.18 atppm.

Also, based on the results of Examples 4 and 6, it was checked that itis possible to set the IL to be less than or equal to 0.05 dB by settingthe Δ value to be −0.15 atppm and 1.19 atppm, which are within a rangefrom −0.20 atppm to 1.21 atppm.

Also, in 0.01 atppm (Example 7) where the Δ value is close to 0 atppm,it was checked that the IL value is decreased more than Examples 1through 6, while it was checked that Example 8 where Δ value is set to0.00 atppm is most preferable compared to Examples 1 through 7 as the ILvalue is a minimum of 0.020 dB.

Also, when the Δ values are again calculated by setting the coefficientα to be 0.850 in Example 1 and to be 0.780 in Example 2, the Δ valuesrespectively become 3.36 atppm and −0.49 atppm, and thus are outside arange from −0.40 atppm to 3.18 atppm. However, since the Δ values arewithin the range when the coefficient α is 0.815, Examples 1 and 2 areembodiments according to the present invention. Also, when the Δ valuesare again calculated by setting the coefficient α to be 0.780 in Example3 and to be 0.850 in Example 5, the Δ values respectively become −0.32atppm and 1.40 atppm. Accordingly, in Examples 3 and 5, the Δ values arealways set from −0.40 atppm to 3.18 atppm even when the coefficient α isset to any value within the range of 0.815±0.035 (that is, 0.780 to0.850). Thus, it is concluded that Examples 3 and 5 are more preferablethan Examples 1 and 2.

Also, when the Δ values are again calculated by setting the coefficientα to be 0.780 in Example 4 and to be 0.850 in Example 6, the Δ valuesrespectively become −0.26 atppm and 1.31 atppm, and thus are outside arange from −0.20 atppm to 1.21 atppm. However, since the Δ values arewithin the range when the coefficient α is 0.815, Examples 4 and 6 areembodiments according to the present invention. Also, when the Δ valueis again calculated by setting the coefficient α to be from 0.780 to0.850 in Example 7, the Δ value is from −0.07 atppm to 0.08 atppm.Accordingly, in Example 7, the Δ value is always set from −0.20 atppm to1.21 atppm even when the coefficient α is set to any value within therange of 0.815±0.035 (that is, 0.780 to 0.850). Thus, it is concludedthat Example 7 is more preferable than Examples 4 and 6.

Comparative Examples 1 and 2

Garnet single crystals of Comparative Examples 1 and 2 were manufacturedunder the same conditions as Examples 1 through 8, except that Δ valuesare changed by changing Mn concentration and Pt concentration in thegarnet single crystals. Also, the manufactured garnet single crystalswere evaluated like Examples 1 through 8. The Mn concentration and thePt concentration are set such that the Δ value is less than −0.40 atppmin Comparative Example 1 and is more than 3.18 atppm in is ComparativeExample 2.

It was checked, by Comparative Example 1, that the IL value is higherthan 0.10 dB when the Δ value is −0.50 atppm, i.e., lower than −0.40atppm.

Also, it was checked, by Comparative Example 2, that the IL value ishigher than 0.10 dB when Δ value is 3.43 atppm, i.e., higher than 3.18atppm.

Also, when the Δ value is again calculated by setting the coefficient αfrom 0.780 to 0.850 in Comparative Example 1, the Δ value becomes from−0.59 atppm to −0.41 atppm, and thus is outside a range from −0.40 atppmto 3.18 atppm. In other words, it is concluded that Comparative Example1 does not follow the present invention since the Δ value is alwaysoutside the range from −0.40 atppm to 3.18 atppm even when thecoefficient α is set to any value in the range of 0.815±0.035 (that is,0.780 to 0.850) in Comparative Example 1.

Similarly, when the Δ value is again calculated by setting thecoefficient α from 0.780 to 0.850 in Comparative Example 2, the Δ valuebecomes from 3.19 atppm to 3.66 atppm, and thus is outside a range from−0.40 atppm to 3.18 atppm. Accordingly, it is concluded that ComparativeExample 2 does not follow the present invention.

Also, setting of the M concentration in the garnet single crystal may bechanged by changing an input amount of M material powder to thecrucible. A ratio of M contained in the garnet single crystal grown fromthe amount input to the crucible has a constant value. Accordingly, bycontrolling the input amount to the crucible according to the ratio, theM concentration may be within a desired range.

In order to set the Pt concentration in the garnet single crystal to adesired range, oxygen concentration of the growth atmosphere may becontrolled.

An optical device of any one of an optical isolator, an opticalcirculator, an optical attenuator, a Faraday mirror, a current sensor, amagnetic field sensor, and a magnetic optical switch manufactured byincluding the garnet single crystal does not adversely affect theenvironment since the garnet single crystal included in the opticaldevice does not include lead.

1. A Bi-substituted rare earth iron garnet single crystal which has acomposition of R3-xBixFe5-wAwO12, wherein R denotes one or more types ofrare earth elements selected from a group consisting of Y, Eu, Gd, Ho,Yb, Lu, Nd, Tm, La, Sm, Dy, Er, Ce, and Pr while definitely comprisingGd, A denotes one or more types of elements selected from a groupconsisting of Ga, Al, In, Sc, Co, Ni, Cr, V, Ti, Si, Ge, Mg, Zn, Nb, Ta,Sn, Zr, Hf, Pt, Rh, Te, Os, Ce, and Lu, 0.7<x≦1.5, and 0<w≦1.5, does notcontain Pb and contains Pt, and additionally contains Mn or at least oneGroup 2 element, wherein, when M denotes Mn or at least one Group 2element, [M] denotes M concentration (atppm) and [Pt] denotes Ptconcentration (atppm) in the Bi-substituted rare earth iron garnetsingle crystal, and a relational expression Δ of [M] and [Pt] isrepresented by Equation 1 below:Δ=(α×[M])−[Pt],  [Equation 1] wherein a coefficient α is set to anyvalue within a numerical range of 0.815±0.035 and Δ is set from −0.40atppm to 3.18 atppm.
 2. The Bi-substituted rare earth iron garnet singlecrystal of claim 1, wherein Δ is set from −0.20 atppm to 1.21 atppm. 3.The Bi-substituted rare earth iron garnet single crystal of claim 1,wherein Δ is set to 0 atppm.
 4. A method of manufacturing aBi-substituted rare earth iron garnet single crystal, the methodcomprising: putting a solvent which does include at least Bi2O3 and doesnot include a lead compound, and does include Fe2O3, Gd2O3, and a soluteaside from Fe2O3 and Gd2O3 into a crucible formed of a Pt material;putting a mixture of any one of MO, MO2, and M2O3, wherein M denotes Mnor at least one Group 2 element, into the solvent; and growing aBi-substituted rare earth iron garnet single crystal on a nonmagneticgarnet crystal substrate, wherein, in the Bi-substituted rare earth irongarnet single crystal, when [M] denotes M concentration (atppm) and [Pt]denotes Pt concentration (atppm) in the Bi-substituted rare earth irongarnet single crystal, and a relational expression Δ of [M] and [Pt] isrepresented by Equation 2 below:Δ=(α×[M])−[Pt],  [Equation 2] wherein a coefficient α is set to anyvalue within a numerical range of 0.815±0.035 and Δ is set from −0.40atppm to 3.18 atppm.
 5. The method of claim 4, wherein Δ is set from−0.20 atppm to 1.21 atppm.
 6. The method of claim 4, wherein Δ is set to0 atppm.
 7. The method of claim 4, wherein the Bi-substituted rare earthiron garnet single crystal is grown under an inert gas atmosphere. 8.Any one of an optical isolator, an optical circulator, an opticalattenuator, a Faraday mirror, a current sensor, a magnetic field sensor,and a magnetic optical switch comprising the Bi-substituted rare earthiron garnet single crystal of claim 1.